The technology in this application relates to optical measurement apparatus and techniques.
FIG. 1 shows a schematic block diagram of an interferometric sensor arrangement 10. An input-output fiber 11 (e.g., a single mode fiber) conducts light from a light source, e.g., a laser source 12, via a coupler 14 to an interferometric sensor 13 (e.g., an EFPI sensor or transducer). A detector 16 detects light reflected back from the sensor 13 via the coupler 14 over fiber 15 (e.g., a single mode fiber 15).
Extrinsic Fabry Perot Interferometer (EFPI) sensors are based on the change in the optical length of a low-finesse Fabry-Perot cavity with respect to an applied measurand. FIG. 2 shows an example EFPI strain gauge sensor 10 that includes an optical fiber 11 inserted into one end of a silica capillary tube 21 used to add structure and prevent debris from entering a cavity or gap 24 formed between the end of the optical fiber and a reflective surface inserted into the other end of the capillary tube. The cavity or gap 24 is formed between the flat endface of the transmitting fiber 11 indicated at reference reflection (R1) and a sensing reflection (R2) surface appropriate for the application, which in this example is the surface of a fiber or reflective object 22 is inserted into the opposing end of the capillary tube 21.
The width of the Fabry-Perot cavity 14, referred to as A gap, is measured by interrogating the sensor 10 with a light source which reflects off both the fiber endface (R1) and the surface (R2) of the transducer 16 and interpreting the resulting interference pattern. When the light arrives at the source fiber end-face, a portion is reflected off the interface caused by differing indices of refraction between the fiber and the transparent media (R1) and the remaining light propagates through a cavity or gap with a second reflection occurring at the media/fiber interface (R2). The distance between R1 and R2 is same as the length of the gap and is one half of an optical path length. In an interferometric sense, R1 is the reference reflection, and R2 is the sensing reflection. These reflective signals interfere constructively or destructively based on wavelength and the optical path length difference between the reference and sensing fibers. Small movements from environmental or other physical forces cause a change in the cavity or gap length causing a phase difference between the sensing and reflecting waves producing interference patterns called “fringes.” The sensitivity to changes in gap length is proportional to the visibility of the interference fringes reflected back into the input fiber. Example interference fringes (intensity v. optical frequency) are shown in FIG. 3A for a 60 μm gap and in FIG. 3B for a 120 μm gap; the optical frequency spacing of the interference fringes is inversely proportional to the gap width.
EFPI technology may be used to monitor a wide variety of parameters in various environments (including harsh environments) such as strain, temperature, pressure, shear, acceleration, electrical/magnetic field, radiation magnitude and radiation types, humidity, chemical constituents, and any other measurable parameters (sometimes called the measurand). Example advantages of EFPI sensors include high temperature operation, small size, and immunity to electrical noise.
EFPI sensors may be analyzed using interrogation systems generally falling into one of two categories of systems: 1) high update rate interrogation systems that make fast relative measurements but have less absolute accuracy, and 2) low update rate absolute interrogation systems that provide more accurate results using a wide range of wavelength data requiring more extensive processing and interrogation time than the high update rate interrogation systems, which reduces the time between measurements.
Wide-spectrum optical emitters, which may be white-light sources such as light emitting diodes (LEDs) or highly coherent light sources such as swept-wavelength lasers, can produce absolute measurements of the optical path length of the Fabry-Perot cavity or gap. Interrogation systems based on white-light optical sources launch a broad range of optical frequencies into the sensing fiber at the same time. Reflected data is collected with a spectrometer to produce a measurement interference signal intensity vs. laser optical frequency. The speed of an interrogation system based on a white-light optical source is limited by the required integration time of the spectrometer, which is dependent on overall signal intensity and range and may typically be on the order of 1 ms to produce a measurement.
EFPI sensors may also be interrogated using tunable laser sources with wide tuning ranges. In this scheme, the laser's optical frequency is swept across a range. As the laser's frequency changes, the interference signal's intensity varies sinusoidally. This sinusoidal signal is collected at a photodetector and converted from the time domain to the spectral domain using the known rate of laser frequency sweep. The speed of data collection is limited by the sweep range and rate of the tunable laser. The speed of an interrogation system based on typical high-speed tunable lasers may be approximately on the order of 50 ms to produce a measurement.
Amplitude-based approaches can increase the speed of EFPI interrogation. Amplitude-based schemes do not directly generate wide wavelength ranges of spectral domain data. Using one or more narrowband lasers, an amplitude-based scheme monitors the change in reflected signal amplitude(s) vs. time using simple photodetectors. By simplifying the detection scheme—the lasers are continuously emitting, and the photodetectors are continuously collecting data—amplitude-based interrogators can achieve high update rates. A primary limitation of measurement speed is the bandwidth of the photodetector and acquisition electronics.
A simple amplitude-based interrogator uses a fringe-counting scheme. A single, narrowband laser emits light at a fixed frequency for interrogating an EFPI sensor. When the sensor's gap changes at a constant rate, the intensity of the return signal varies sinusoidally with time. Assuming a monotonic change in the measurand during the time period of interest, the total change in the measurand may be determined in part by counting the number of times the intensity reached a maximum value (the number of fringes produced by the total change in the gap length) plus the fraction of the next fringe generated. This fringe counting method provides only a relative, rather than an absolute, measurement of gap displacement. In addition, the accuracy of this relative measurement suffers when the detected intensity is in the vicinity of a maximum or minimum. This is due to a lower rate of intensity change with phase at the extrema of a sine wave as compared with the linear, rapidly changing regions of the waveform.
What is needed is a system that is capable of producing highly accurate absolute measurements of an interferometric (e.g., EFPI) sensor while also obtaining high update rates.